How To Steer Sound Using Light

The ability to create phonons and then steer them using laser beams could lead to a new generation of applications, say physicists

July 16, 2012

Zap an optical fibre with a couple of laser beams and the resulting interference pattern causes an interesting effect–it squeezes the material, an effect known as electrostriction. This creates a compression wave called a phonon, a packet of sound, which travels along the fibre.

Not to be outdone, phonons also influence light because they change the refractive index of the material. This bends light and alters its frequency, an effect known as Brillouin scattering.

After that, things get complicated. This mechanism sets in train a complex set of feedback effects in which photons generate phonons which influence the photons and so on.

The problem is understanding what’s going on. The ability to influence sound with light, and vice versa, could have interesting applications. But without an accurate model of this phenomenon, it’s hard to exploit.

That looks set to change. Until now, physicists have sought to understand the phenomenon by assuming the phonons have a particular form and working out how this influences the incident light. In other words, they ignore feedback effects.

Today, Jean-Charles Beugnot and Vincent Laude at Université de Franche-Comté in Besançon, France, take a more detailed at look the problem.

For the first time, these guys simulate how light generates phonons inside an optical fibre and how the phonons then interact with the light that generated them. They then test their ideas by measuring the way phonons scatter light in two types of fibre.

Their conclusion has interesting implications. They say that the light ends up guiding the phonons that it creates. In other words, it’s possible to create and then steer sound using light. “The phonon wavepacket generated via [electrostriction] is naturally guided by the light that gave it birth,” say Beugnot and Laude.

These guys aren’t very forthcoming with potential applications but it doesn’t take much imagination to speculate. Engineers already use light to reduce vibrations in mirrors, cooling them close to absolute zero.

Beyond this,the most exciting application is in information processing. In this field, there is growing interest in controlling phonons because they are essentially noise generated by heat.

Controlling phonons would allow them to be steered away from sensitive areas. More ambitious still is the possibility of computing with phonons, with various groups around the world working on phonon diodes and transistors, the building blocks of logic gates.

Progress so far is tentative but that could soon change. Other suggestions in the comments section please.